The amount of oil and/or gas in a subterranean reservoir will determine whether developing the reservoir and bringing the oil or gas to the surface is economical. Sedimentary reservoir rocks comprise a porous medium. The pore spaces in these rocks are typically fluid-filled, usually with a brine that is electrically conductive and relatively dense. Hydrocarbon fluids are electrically resistive and less dense than brine. During hydrocarbon emplacement in the pore spaces of a reservoir rock, hydrocarbons will typically accumulate by buoyantly displacing brine from the pore spaces. Rocks filled with oil and/or gas are more resistive than brine-filled rocks. The resistivity contrast between brine-filled and hydrocarbon-filled rocks provides a means of remotely sensing the presence of hydrocarbons using instruments lowered into a well from the surface.
A remote sensing technique used to quantify the amount of oil or gas in a reservoir is called resistivity logging. Resistivity logging instruments respond primarily to the reciprocal of resistivity, or conductivity, and these instruments are referred to in the remainder of this document as “conductivity logging instruments.” Formation conductivity, inferred from conductivity logging instrument responses, is used in conjunction with laboratory conductivity measurements to estimate whether commercial quantities of hydrocarbons exist in potential hydrocarbon reservoirs. Further, formation conductivity is commonly used (with other measurements) to quantitatively evaluate the amount of hydrocarbons in the reservoir. Persons skilled in the art can produce hydrocarbons after a hydrocarbon reservoir has been located using formation conductivity or other means.
The electrical conductivity property at any given point in a reservoir will in general vary according to the direction in which the property is measured. In a specified small volume of the reservoir, the conductivity measured in a nominal horizontal (i.e., parallel to sedimentary bedding plane) direction typically will differ from the conductivity measured in a nominal vertical (i.e., perpendicular to sedimentary bedding plane) direction. In many reservoir rocks, there will also exist two different values of conductivity oriented in mutually orthogonal directions in the nominal horizontal plane.
In general, at every point in the conductive medium there are three mutually orthogonal conductivity values that can be taken as defining a Cartesian coordinate system with its orthogonal axes being characterized by the conductivity values. The axes of the Cartesian coordinate system defined by the three mutually orthogonal conductivity values are called the principal components, or principal axes, of the conductivity tensor. The principal axes of the conductivity tensor are further characterized by their orientation with respect to the earth. This orientation can be described by three angles that measure the orientation of the conductivity tensor with respect to axes directed north-south, east-west, and up-down. Thus, at each point in the medium, six numbers are required to specify the conductivity values and their orientation (i.e., three angles and three magnitudes of the conductivity tensor). The conductivity tensor is specified by these six numbers. When the three mutually orthogonal conductivities are the same, there is no preferred direction in the medium and the three angles can be considered zero. For such cases, the conductivity tensor reduces to a single number representing conductivity. When the medium conductivity is represented by a single number, the medium is isotropic. Otherwise, the medium is anisotropic. Most sedimentary formations are anisotropic.
This anisotropic complexity has not been recognized in conventional formation evaluation. The differences in the horizontal conductivity components in vertical wells that penetrate horizontally bedded sedimentary rocks have been assumed to be negligible. Further, the vertical component of conductivity has been assumed to be either the same as the horizontal component of conductivity or else assumed not to excite a response in conductivity logging instruments operated in vertically drilled wells. Laboratory conductivity measurements have always been oriented to sample a nominally horizontal, but otherwise unspecified, component of a rock's conductivity. Therefore, formation resistivity at a point in a hydrocarbon-bearing reservoir rock has typically, but inaccurately, been characterized by a single number (known by the misnomer Rt, for “true” resistivity) rather than the six independent numbers comprising the elements of a conductivity tensor.
The introduction of horizontal drilling techniques and their widespread use has resulted in instruments and techniques developed for vertical wells being applied in deviated, highly deviated, and horizontal wells. These techniques are often unsuccessful because of the inappropriateness of these instruments and techniques for use in any environment except vertical wells or isotropic media.
The poor response characteristics of conventional conductivity logging instruments to anisotropic formations is typified by “low resistivity pay.” In vertical wells drilled through hydrocarbon bearing sand-shale reservoirs, an induction instrument response can be dominated by highly conductive shales. Despite commercial quantities of high resistivity hydrocarbon-bearing sandstones surrounding the instru-, ment in the reservoir, the apparent resistivity does not unambiguously reflect the presence of these hydrocarbons. Thus, this kind of reservoir has been called “low resistivity pay” in the oil and logging industries. Conversely, compared to the vertical wells, the apparent resistivity responses of horizontal wells are too high in the presence of anisotropy. The physical mechanism for these anomalously low values of apparent conductivity in horizontal wells is more complicated than for low resistivity pay in vertical wells and has only recently been understood. (Weiss, Chester J., Lu, Xinyou, and Alumbaugh, David, 2001, Visualization of eddy currents induced in an electrically anisotropic formation, Petrophysics, Vol. 42, No. 6.).
In isotropic formations, the response of conventional conductivity logging instruments can be used to accurately estimate formation conductivity. Anisotropic formations have a vertical conductivity component that is different from the horizontal conductivity component (or components). Conventional instruments respond only to the horizontal conductivity if they are deployed in a vertical wellbore. However, in the general case of a fully anisotropic medium and a deviated well, conventional instruments cannot be used to uniquely infer or accurately estimate any of the six components of the conductivity tensor.
An induction logging instrument's transmitters induce magnetic fields in its receivers. The magnetic field at each receiver due to each transmitter is the sum of two components. For specificity, consider only the magnetic field induced in a single receiver excited by a single transmitter. One magnetic field component is called the primary field and represents the voltage induced in the receiver by the magnetic field produced by the transmitter itself. This component exists even when the instrument is immersed in a zero-conductivity medium such as a vacuum or air. The primary field does not contain any information from the environment surrounding the instrument, and is not considered part of the “signal.” In a conductive medium the primary field is still induced in the space surrounding the receiver, but there is in addition to the primary field another magnetic field component. The second component of the field arises due to eddy currents induced to circulate in the conductive formation. This “secondary” component of the magnetic field at the receiver is a monotonically increasing function of the conductivity in the environment surrounding the instrument and is considered to be the “signal” to be detected in hydrocarbon logging operations.
The primary component of the field is much larger than the secondary field making detection of the secondary field in the presence of the primary field problematic. Thus, the primary field is always canceled in practical instruments by some means. The primary field is usually cancelled by the use of appropriately located “bucking” coils designed to introduce a signal at the receiver that is oppositely directed but equal in magnitude to the primary field. When the primary field is properly “bucked out” only the signal from the secondary field remains in the receiver.
The primary field in the context of induction logging is sometimes referred to as the “direct” or “direct-coupled” field or signal. However, in the description of triaxial induction instruments the term “direct-coupled” is given a different meaning, as shall be defined in the next paragraph. In the remainder of this description the primary fields are considered to have been compensated by appropriate “bucking coils,” or other means. Only signal, or secondary field, components are discussed. Accordingly, as used herein, “signal” will always refer to the secondary field that arises in response to transmitter-induced eddy currents circulating in the conductive formation, and further, the term “direct-coupled” will never refer to the primary field, and will always be used in the sense to be defined below.
New induction logging instrument technologies (e.g., U.S. Pat. Nos. 5,999,883 and 5,999,884) are capable of recording magnetic field data that, under favorable conditions, permit the inference of all three principal components of the conductivity tensor. The new technology as disclosed in these patents differs from the prior art by using transmitters and receivers oriented transverse to the instrument axis in addition to the usual coaxial transmitters and receivers, resulting in a total of three mutually orthogonal transmitters, and a corresponding set of three mutually orthogonal receivers.
Each transmitter is paired to a similarly oriented receiver located at the same distance from the transmitter. The signal (i.e., the secondary field resulting from transmitter-induced eddy currents in the surrounding formation) induced in the receiver by its associated transmitter is referred to herein as a “direct-coupled signal.” If a signal is induced in the receiver by either of the other two transmitters it is referred to herein as a “cross-coupled signal.” The new technology can record the magnetic field data that permits the inference of the full conductivity tensor only if the instrument axes are parallel to the principal axes of the conductivity tensor. In such cases, all of the magnetic induction tensor components not on the main diagonal of the matrix representing the tensor, and corresponding to cross-coupled transmitter-receiver dipoles, are equal to zero, and hence are known a priori. Therefore, the preferred use of this new technology has been in vertical wells drilled in formations having approximately equal horizontal conductivity principal components. Consequently, the cross-coupling is approximately equal to zero.
As shown in FIG. 1, conventional induction logging instrumentation employs magnetic dipole sources (transmitters) 1, which are oriented coaxially with the instrument axis 5. The detectors or receivers 3 are also oriented coaxially with the logging instrument axis 5. The conventional induction logging array takes many forms but all comprise one or more coaxially arrayed transmitters 1 and one or more coaxially arrayed receivers 3. The mutual axis of the transmitters and receivers dipoles is coaxial with the axis 5 of the instrument on which they are conveyed. Further, the mutual axis of the dipoles is approximately coaxial with the borehole where the instrument is deployed.
The instruments operate by inducing eddy currents in earth formations by means of time-varying magnetic fields generated by their transmitters. As shown in FIG. 1, the induced eddy currents 2 flow in ground loops that tend to circulate in planes that in isotropic formations are substantially perpendicular to the axis of the instrument 5. This current distribution is also characteristic of horizontally-lying anisotropic formations penetrated by vertical well bores. In each case voltages related to the magnitude of the eddy currents are then induced in an instrument's receiver coils. The voltages are then sampled and recorded using equipment known in the art.
The conventional interpretation model for this instrumentation is a homogeneous isotropic medium. The secondary magnetic fields arising from eddy currents induced in a formation are detected by the receiver coil or coils and are related to the formation conductivity. In homogenous isotropic media, this relation is well known and these instruments can make an accurate determination of formation conductivities. However, conventional instruments are often used in an attempt to quantify the conductivity of formations that are inhomogeneous and anisotropic.
Conventional induction instrumentation cannot quantify the conductivity distribution in such media because the axial dipole transmitters and receivers are not capable of sampling the full magnetic induction data space. When the responses of conventional induction instruments to a heterogeneous, anisotropic medium are interpreted without recognition of the nature of the medium, the conductivity is misquantified. Therefore, fluid volume interpretations based upon this conductivity estimate are often misestimated.
As described above, the newest type of induction instruments, triaxial induction instruments, introduce the use of source and receiver magnetic dipoles mounted transverse to the instrument axis in addition to the conventional coaxially-mounted dipoles. FIG. 2A is a diagrammatic illustration of the transmitter 7 and receiver arrays 9 for a triaxial induction logging instrument. The circles are intended to indicate three mutually orthogonal loops of wire (solenoids) sharing the same center point (collocated magnetic dipoles). If electric current flowed in the wires, magnetic fields would arise in the space surrounding the wires. The magnetic dipole moments of these current loops (indicated by arrows 11, 12) provide a convenient alternative graphical representation of the sources (transmitters) 7 and receivers 9.
An “equivalent point magnetic dipole” is an imaginary point magnetic dipole that would produce substantially the same magnetic field at a receiver's location as the actual transmitter coil. The term “center” as used in the text refers to the point inside a coil where an equivalent magnetic dipole could be located.
In FIG. 2A the transmitter array center and receiver array center are spaced apart a distance L. FIG. 2B illustrates a similar transmitter and receiver array except that the transmitter coils 30 have been separated and each transmitter coil is spaced a distance L from the corresponding similarly separated receiver coils 37. In FIG. 2A, both the collocated direct- and cross-coupled coils have the same spacing L. However, in FIG. 2B, illustrating the separated coils, the direct-coupled coils remain spaced at a determined distance of L, but the cross-coupled components are not spaced at the determined distance L.
In the newest induction logging instrument designs, the sources and receivers are arranged in three mutually orthogonal directions. One direction is in the usual coaxial-to-the-instrument-axis orientation and two mutually orthogonal directions are in the transverse-to-the-instrument-axis plane. In principle there are nine possible signal components. Conventional axial-dipole induction instrumentation samples only the axial component of the magnetic field induced by the axially-directed transmitter. This is designated the ZZ direct-coupled response, indicating the response of the z-directed receiver 16 to the z-directed transmitter 15 (See FIG. 2B).
In principle, the new generation of instrumentation should be able to detect the x component 20 and y component 18 of magnetic field at the receiver array due to the excitation of the z source component 15. These are designated the ZX and ZY cross-coupled responses. In isotropic media, the z-component of the source cannot excite a signal in x- and y-directed receiver coils on the axis of the instrument and these cross-coupled signal components will be zero. In anisotropic formations, this is generally not true. Likewise, the horizontal transmitters 17, 19 are coupled to each of the receivers 16, 18, 20. These responses are designated XX, XY, XZ, YX, YY, and YZ. Thus, a total of nine signals characterize the full magnetic induction response data space. However, the principle of electromagnetic reciprocity applies to the transmitter and receiver arrays of this instrument. Therefore the response pairs XY and YX, XZ and ZX, YZ and ZY are the same, thus, for example, making the separate detection of XY and YX unnecessary since detection of one provides the value of the other. This is also true for the remaining two cross-coupled pairs of signals, XZ and ZX, YZ and ZY.
In view of electromagnetic reciprocity, the full magnetic induction data space comprises six independent magnetic field response pairs or six magnetic field components. When three mutually orthogonal transmitter coils are collocated and the three receivers are similarly arrayed collocally, then the six simultaneously (or almost simultaneously) excited and recorded magnetic field components comprise a magnetic induction tensor.
The magnetic induction tensor is a complete description of the magnetic field response at the receiver location due to excitation of a triaxial transmitter at the transmitter location. Ideally, the magnetic induction tensor contains information regarding the conductivity of the medium between the transmitter and receiver. Accordingly, the tensor is conventionally associated with the point in the wellbore (and therefore the earth) midway between the transmitter and receiver. The magnetic induction tensor may be used to deduce, (e.g., using mathematical inversion or other means) the principal components of the conductivity tensor, and the orientations of these principal components. The components of the conductivity tensor can subsequently be used to estimate the hydrocarbon content of the reservoir. Methods to transform the magnetic induction tensor components into the components of the apparent conductivity tensor are known in the art. (Zhdanov, M., Kennedy, W. D., Cheryauka, A. B., Peksen, E., 2001, Principles of tensor induction well logging in a deviated well in an anisotropic medium, 2001 Transactions of Society of Professional Well Log Analysts, Houston Tex.; Zhdanov, M., Kennedy, W. D., Peksen, E., Foundations of tensor induction well-logging, Petrophysics, vol. 42, no. 6., pp 588-610.). Methods for estimating formation water saturation from components of the apparent conductivity tensor are also known (Kennedy, W. D., Herrick, D. C., Yao, T., 2001, Calculating water saturation in electrically anisotropic media, Petrophysics, vol. 42, no. 2, pp. 118-136).
The conventional implementation of this technology as disclosed by Gupta, et al. in U.S. Pat. Nos. 5,999,883 and 5,999,884 does not employ collocated transmitters and receivers. A possible reason for the lack of collocated transmitters and receivers in the Gupta instrument include difficulties in the manufacturability of such transmitter and receiver arrays. Whatever the reason, for the instrument described in the Gupta patents and illustrated in FIG. 2B the transmitter array is separated into three separate coils spaced along the instrument axis. The receiver array is also separated into three independent, non-collocated coils such that the spacings between corresponding transmitter and receiver of each pair of coils are equal. This arrangement of transmitters and receivers allows the detection of the three direct-coupled magnetic field components referred to above as XX, YY, and ZZ.
The three direct-coupled magnetic field components comprise three of the six independent components of a magnetic induction tensor characteristic of the formation at a point, and have a representation as the diagonal elements of a 3×3 matrix. The off-diagonal elements of the matrix represent the cross-coupled elements of a magnetic induction tensor. The cross-coupled components generated by the Gupta instrument coil arrangement are not spaced at the correct distances to be components of the same tensor as the direct-coupled signals.
There are no provisions in the Gupta instrument patents to correctly obtain these missing cross-coupled components of the magnetic induction tensor. The cross-coupled components are approximated based upon the responses of the coils that are contained in the Gupta instrument array, but these coils are improperly spaced to detect the missing components of the magnetic induction tensor. Cross-coupled coils provided for the purpose of detecting cross-coupled signals are also improperly spaced to detect the missing components of the magnetic induction tensor. Therefore, the Gupta instrument does not respond directly to the full data space necessary to detect and resolve the conductivity tensor.
When the axis of the Gupta magnetic induction instrument is parallel to one of the principal axes of the conductivity tensor, then it is known a priori that the cross-coupled terms are zero. For all such cases, the instrument fully resolves the three direct-coupled magnetic induction tensor components. In such cases, the cross-coupled magnetic induction tensor components are assumed to be known (i.e., assumed to be zero) a priori and the full magnetic induction tensor data space is assumed to have been sampled and the conductivity tensor can then be deduced. Otherwise, the conductivity tensor cannot be deduced. The Gupta instrument was apparently designed to quantify “low resistivity pay” in vertical wells where the geometrical restrictions mentioned above usually prevail, but the design is not optimized for use in deviated and horizontal wells.
The problem with the current technology is its inability to acquire all six components of the magnetic induction tensor regardless of the relative orientation of the logging instrument and the principal axes of the conductivity tensor. Accordingly, to adequately evaluate electrically anisotropic formations, new instrumentation is necessary to determine the magnitudes and the directions of each of the six components of the magnetic induction tensor at each point in a formation. The present invention satisfies this need.